It is known to provide an alternating current mains supply at either 110 volts or 230/240 volts from a photovoltaic device using an inverter circuit. One standard photovoltaic (PV) panel provides approximately 20 volts DC at around 4.5 amps maximum and this voltage must be stepped up and converted to alternating current to provide a mains output. This is generally done using an inverter constructed from discrete electronic components to convert the low DC input voltage to a high AC output voltage. Alternatively, there may be an initial step to step up the DC voltage before converting it to an AC voltage. An implementation of such a basic arrangement using the Hitachi ECN 3067 integrated circuit and, optionally, the ST Microelectronics L298 integrated circuit is described in “Grid Connected PV Inverter using a Commercially Available Power IC”, A. Mumtaz, N. P. van der Duijn Schouten, L. Chisenga, R. A. MacMahon and G. A. J. Amaratunga presented in October 2002 at the PV in Europe conference in Rome, Italy.
Further prior art can be found in AU 58687, U.S. Pat. No. 6,151,234, AU 2073800, EP 1035640, NL 1011483C, U.S. Pat. No. 4,626,983 A, EP 0628901 A, U.S. Pat. No. 6,603,672 B, JP 2002 354677 A and JP 4 364378 A.
In practice, supply of power to a domestic grid mains is complicated by the need to maintain the quality of supplied power within standard limits, typically determined by regulatory authorities. These may include over-current, under-voltage and over-voltage, and under-frequency and over-frequency conditions. Recommendations for the connection of photovoltaic generators of up to 5 KVA are contained in the Engineering Recommendation G77, produced by the U.K. Electricity Association. It contains details for isolation of a PV inverter from the grid if the operating voltage exceeds the 230V+10% (253V) or −10% (207V). The operating frequency should not exceed 50 Hz+1% (50.5 Hz) or 6% (47 Hz); DC current injection should not exceed 5 mA. A particularly important problem is referred to as “islanding”—that is if the domestic mains grid supply is switched off or tripped, for example to protect equipment and/or maintenance staff, then the PV inverter must also cease to supply power to the grid. According to the aforementioned recommendations the disconnection time for the PV inverter should be less than 5 seconds